DOI: 10.1002/cvde.200606512

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Thin Films of ZrO2 for High-k Applications EmployingEngineered Alkoxide- and Amide-Based MOCVD Precursors**

By Reji Thomas, Raghunandan Bhakta, Andrian Milanov, Anjana Devi,* and Peter Ehrhart*

Ultrathin ZrO2 films were deposited on SiOx/Si in a multiwafer planetary metal-organic (MO)CVD reactor combined with aliquid delivery system. Two different alkoxide-based precursors, [Zr(OiPr)2(tbaoac)2] and [Zr(OtBu)2(tbaoac)2] are com-pared with two amide-based precursors, [Zr(NEt2)2(dbml)2] and [Zr(NEtMe)2(guanid)2]. Growth rate, surface roughness,density, and crystallization behavior are compared over a wide range of deposition temperatures (400–700 °C). In addition,the influence of the solvents, n-butylacetate, toluene, and hexane, is discussed. The best growth results in terms of low temper-ature deposition rate, surface roughness, film density, and carbon content were obtained for the new [Zr(NEtMe)2(guanid)2]precursor. The electrical properties were investigated with metal–insulator–semiconductor (MIS) capacitors. The relative di-electric permittivity was in the range 17–24, depending on the precursor. Compared to standard SiO2 capacitors of similarequivalent oxide thickness, low leakage currents were obtained.

Keywords: High-k dielectrics, Metal–organic precursors, MOCVD, Zirconium oxide

1. Introduction

The technological importance of ZrO2 thin films is dueto the widespread use of this material in oxygen ion con-ductors and sensors, optical coatings, laser mirrors, wear-re-sistant coatings, magnetic recording disks, biomedical andprosthetic coatings. Recently, interest in ultrathin ZrO2

films has increased due to the need to find a replacementfor SiO2 as a gate oxide material for future sub-micronelectronic devices.[1] The thermal stability and wide band-gap (5.65 eV) of this material and its silicates make it a fa-vorite in this field, together with HfO2 and its silicates. Ad-ditionally, Zr is an inseparable part of many functionalperovskites such as Pb(Zr,Ti)O3, BaZrO3, and the integra-tion of these materials into semiconductor devices is onlya matter of time due to the ever-demanding transistorspeed and large memory storage needed to keep pace withMoore’s law.[2]

Thin films of ZrO2 have been fabricated using a variety ofmethods such as sputtering, pulsed laser deposition (PLD),and chemical deposition methods such as MOCVD andatomic layer deposition (ALD).[1] The complex surface to-pography of the CMOS devices, and possible damage to thechannel, together with the need for uniformity over the wa-fer area favor CVD techniques. As amorphous films arepreferred for gate-oxide applications due to their betterelectrical uniformity (spatial invariance of electrical proper-ties), the CVD techniques must be applicable at low tem-peratures. As liquid injection MOCVD is the technique ofchoice for large-scale production it becomes crucial to finda precursor/solvent combination that does not increase thecarbon content in the grown film. Although great progresshas been achieved in the development of MOCVD andALD techniques for the deposition of ultrathin filmsfor gate oxide applications,[1] and in recent precursor devel-opment,[3,4] there is a need for further improvements. Inthis paper we present a comparative study of four novelMOCVD precursors and, additionally, discuss the effect ofthe solvent on the deposition characteristics of ZrO2 films.Electrical properties of the MIS structures will be discussed.

2. Results and Discussion

2.1. Precursors and Solvents

The synthesis and characterization of the precursors,a) zirconium bis(isopropoxide)-bis(tert-butylacetoacetate),[Zr(OiPr)2(tbaoac)2], b) zirconium bis(tert-butoxide)-bis-(tert-butylacetoacetate) [Zr(OtBu)2(tbaoac)2],[5,6] c) zir-

98 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2007, 13, 98–104

–

[*] Prof. A. Devi, Dr. R. Bhakta, A. MilanovInorganic Materials Chemistry GroupLehrstuhl für Anorganische Chemie II, Ruhr-University BochumUniversitätsstr.150, D-44780, Bochum, (Germany)E-mail: anjana.devi@rub.de

Dr. P. Ehrhart, Dr. R. ThomasIFF-Institut für Festkörperforschung andCNI-Center for Nanoelectronic Systems for Information TechnologyForschungszentrum Jülich, 52425, Jülich, (Germany)E-mail: p.erhart@hotmail.de

[**] The authors acknowledge financial support from the DeutscheForschungsgemeinschaft (DFG, CVD-SPP-1119, WA- 908/13-3 andDE-790/3-3). Also, we thank W. Krumpen for the XRF analysis,Dr. U. Breuer for the ToF-SIMS, and H. Haselier for the deposition ofthe Pt electrodes. Last, but not least, we thank Prof. R. A. Fischer andProf. R. Waser for their continuous support.

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conium bis(diethylamide)-bis(di-tert-butylmalonate)[Zr(NEt2)2(dbml)2],[7] and (d) zirconium bis-(ethylmethyl-amido)-bis(N,N’-diisopropyl-2-ethylmethylamidoguanidi-nate) [Zr(NEtMe)2(guanid)2], have been reported pre-viously.[8] The precursor complexes are volatile monomersdisplaying promising thermal properties suitable for CVDapplications. The vaporization and decomposition tempera-tures for the precursors, as deduced from the thermal analy-sis data, are summarized in Table 1. As all precursors aresolids at room temperature (RT) dissolution in organic liq-uids and liquid injection is the preferred technique forlarge-scale production. However, the optimization of theprecursor/solvent combination is an additional challenge. Inaddition to the required high solubility, interactions in thesolution and during deposition must be taken into account,and various solvents, such as n-butylacetate, octane, hexane,and toluene, have therefore been considered. The testedprecursor/solvent combinations are included in Table 1. Asbutylacetate was a reasonable solvent in our previousstudy of similar Ti complexes,[9] and was, in addition, com-patible with commercial Sr and Ba precursors, we generallystarted with this solvent. This solvent was suitable for[Zr(OiPr)2(tbaoac)2] but somewhat surprisingly for [Zr(Ot-

Bu)2(tbaoac)2] poor solubility in the same solvent was ob-served. Hence, these two precursors were compared afterdissolution in hexane.[10] However, hexane resulted in high-er crystallization temperatures and higher carbon contentfor the [Zr(OiPr)2(tbaoac)2] precursor. Hence, the avail-ability of a proper solvent might improve the otherwise in-ferior performance of [Zr(OtBu)2(tbaoac)2]/hexane as dis-cussed below. Similar observations were also made withcomparable Hf precursors, [Hf(OiPr)2(tbaoac)2] was notproperly soluble in octane and butyl acetate but readily dis-solved in hexane and toluene.

2.2. Film Growth and Microstructural Evolution

ZrO2 films were deposited in an AIXTRON 2600G3 Pla-netary Reactor.[11] For the comparative study we used 0.05

molar solutions of the above-mentioned precursors. Theprecursor solution was injected by a TRIJET system with atypical pulse length of 0.8 ms (corresponding to 5 lL ofprecursor solution) and with pulse intervals of 0.32 s.This corresponds to an average precursor flow rate of∼ 1 mL min–1. Deposition experiments were carried out un-der ∼ 1.5 mbar reactor pressure and argon was used as thecarrier gas and O2 as an oxidizer. Si(100) substrates wereused without removing the native oxide layer of ∼ 1 nmthickness.

2.2.1. Growth Characteristics

The growth rates of the ZrO2 films are summarized inFigure 1 in the form of Arrhenius plots. For all precursor/solvent combinations, we observe a plateau for medium de-position temperatures with a maximum growth rate of

3–4 nm min–1. For the alkoxide-based precursors, there issome indication of an exponential dependence in the lowtemperature region (corresponding to an effective activa-

tion energy of ∼ 0.75 eV), presumably due to thekinetically-controlled precursor reactions on thesubstrate surface. For the [Zr(OiPr)2(tbaoac)2]precursor in n-butylacetate and hexane thekinetically controlled region lies between 400–500 °C; in the case of [Zr(OtBu)2(tbaoac)2] pre-cursor in hexane, we observe a similar slope,however the growth rate was lower for thelow temperature range and the linear regionextended to 600 °C. This behavior indicatesthat the [Zr(OtBu)2(tbaoac)2] is the more stablecomplex. For the amide-based precursors thelow temperature growth rates are higher andthe slope is smaller corresponding to aneffective activation energy of ∼ 0.48 eV for[Zr(NEt2)2(dbml)2]; for the guanidinate-based

Chem. Vap. Deposition 2007, 13, 98–104 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cvd-journal.de 99

Table 1. Precursors and Solvents.

Precursor

Vaporizationtemperature

[ °C] [a]

Decompositiontemperature

[ °C]Solvent tested

Best solventfor low Tdep

[Zr(OiPr)2(tbaoac)2] 120 245butylacetatehexane [b]

butylacetate

[Zr(OtBu)2(tbaoac)2] 140 230butylacetate [c]

hexanehexane

[Zr(NEt2)2(dbml)2] 190 240butylacetatehexane [d]

butylacetate

[Zr(NEtMe)2 (guanid)2] 120 250

butylacetate [c]

hexane [d]

toluene

toluene

[a] from thermal analysis (TG) under ambient pressure. [b] High carbon content for low Tdep.[c] Poor solubility (0.05 molar solution not achieved or unstable). [d] Soluble but low boilingpoint solvent which is not suitable for this precursor.

1.0 1.1 1.2 1.3 1.4 1.5 1.60.1

0.5

1

5

10700 600 500 400

Susceptor temperature (ºC)

Gro

wth

rate

(n

m/m

in)

1000/T (K-1)

(a) in- Butyl acetate

(a) in- Hexane

(b) in- Hexane

(c) in- Butyl acetate

(d) in- Toluene

Fig. 1. Temperature dependence of the growth rate of ZrO2 films on Si(100)substrates for 4 different precursors: a) [Zr(OiPr)2(tbaoac)2], b) [Zr(OtBu)2-(tbaoac)2], c) [Zr(NEt2)2(dbml)2], (d) [Zr(NEtMe)2(guanid)2]. For pre-cursor (a) the influence of the solvents (butylacetate vs. hexane) is alsoshown.

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precursor, [(Zr(NEtMe)2(guanid)2], the region, which canbe described by a unique activation energy, is very limitedand there is a steep drop in growth rate at the lowesttemperature (350 °C). At higher substrate temperatures(> 650 °C) we again observe influences of precursor typeas well as solvent: the growth rate reduces for the[Zr(OiPr)2(tbaoac)2] precursor much faster in n-butylace-tate than in the hexane solvent. For [Zr(OtBu)2(tbaoac)2]the growth rate was almost twice that of the maximumgrowth rate obtained from [Zr(OiPr)2(tbaoac)2]. The gua-nidinate-based precursor, which had the highest growthrate at low T, shows a decrease in the growth rate startingat the lowest T of ca. 525 °C.

Hence, all precursors have some pros and cons, and thechoice depends on the details of the application. From thegrowth prospect for amorphous films at low temperatures(< 500 °C), the amide-based precursors, especially the gua-nidinate-based precursor, [(Zr(NEtMe)2(guanid)2] seem toshow superior properties. On the other hand, [Zr(OtBu)2-(tbaoac)2] seems to be the most stable complex and showsno decrease in the growth rate up to 700 °C. For the exam-ple of [Zr(OiPr)2(tbaoac)2] there is also an indication thathexane becomes the better solvent at high temperatures(> 600 °C).

2.2.2. Surface Morphology

Surface morphology of the films was studied using atom-ic force microscopy (AFM) and Figure 2 shows the exam-ple for an amorphous film of 4 nm thickness, which was de-posited at 400 °C from [(Zr(NEtMe)2(guanid)2]. These

films were homogeneous over all length scales and had atypical root mean square (rms) roughness of 0.13 nm. Theroughness of the films, as a function of deposition tempera-tures, is summarized in Figure 3 for various precursor/solvent combinations. Film thickness was in the range7–15 nm and within this range there was no systematicchange of the roughness with the film thickness. For thealkoxide-based precursors there is no dramatic change ofthe roughness with deposition temperature within the givenscatter of the data and, remarkably, there is no significant

change at the transition from amorphous films to crystal-line films, which is observed between 400 and 550 °C asdiscussed below. This similarity in the roughness of amor-phous and crystalline films is also observed for the guanidi-nate-based precursor as crystallization is observed below400 °C for this precursor, and the increased roughnessabove 550 °C might therefore be related to pre-reactionsand the reduced growth rate as shown in Figure 1. Only forfilms from the malonate-based precursor was a roughening,along with the amorphous to crystalline transition at∼ 450 °C, observed. Nevertheless, very smooth crystallinefilms can be obtained by deposition of amorphous films at400 °C and crystallization by post-deposition annealing, asdiscussed previously.[7] Although there are only small dif-ferences in roughness for low T deposition, the guanidi-nate-based precursor again shows the best values.

2.2.3. Crystal Structure and Density

X-ray diffraction (XRD) studies were performed onthe films deposited on Si(100) to study the onset of crystal-lization with susceptor temperature or post-depositionannealing. Figure 4 shows, as an example, the XRDpatterns of the films deposited at 600 °C from the[Zr(OiPr)2(tbaoac)2]–butyl acetate combination. The peakaround 2h = 30.3°, may correspond to the tetragonal and cu-bic (111) reflection, but the rather thickness-independentpeak width of the reflections around 2h = 35° and 50° sug-gests an overlap of 002 and 200, and 202 and 220 reflectionsand the tetragonal structure is assigned to the ZrO2 films.Table 2 shows a summary of the onset of crystallization forvarious precursor/solvent combinations, and of the post-de-position annealing temperature for crystallization of as-de-posited amorphous films. Crystal structures of films fromthe different precursor/solvent combinations remained thesame and were polycrystalline in nature. Crystallizationtemperature may depend on film thickness, especially forfilms below 20 nm thickness, as discussed in detail for

100 www.cvd-journal.de © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2007, 13, 98–104

5µmx5µm 1µmx1µm

Fig. 2. AFM plane view of the surface (5 lm × 5 lm) of a 4 nm film depos-ited at 400 °C on Si(100) from [Zr(NEtMe)2(guanid)2] in toluene, showingvery smooth surface with an rms roughness about 1.3 Å.

350 400 450 500 550 600 6500.0

2.0

4.0

6.0

8.0

10.0

RM

S r

ou

gh

nss (

Ao)

Susceptor temperature (ºC)

(a) in- Butyl acetate

(a) in- Hexane

(b) in- Hexane

(c) in- Butyl acetate

(d) in- Toluene

Fig. 3. Dependence of the rms roughness on the susceptor temperaturefor ZrO2 films on Si(100) substrates for 4 different precursors:a) [Zr(OiPr)2(tbaoac)2], b) [Zr(OtBu)2(tbaoac)2], c) [Zr(NEt2)2(dbml)2],d) [Zr(NEtMe)2(guanid)2]. For precursor (a) the influence of the solvents(butylacetate vs. hexane) is also shown. Typical film thickness was in therange 7–15 nm.

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HfO2 films,[12] therefore films of typically 10 nm thicknesswere selected. However, some variations arise as identicalfilms were not always available. It can be seen from this ta-ble that this crystallization temperature depends stronglyon the type of solvent used rather than on the precursor it-self. Films deposited at 400 °C were amorphous, and at450 °C crystalline reflections started to appear for all pre-cursors except [Zr(OtBu)2(tbaoac)2]. It seems this delayedcrystallization is the result of the change in solvent tohexane as a similar behavior was also observed for[Zr(OiPr)2(tbaoac)2] in hexane.

This observation is somewhat related to the densificationof the films observed by X-ray reflectivity (XRR), which issummarized in Figure 5. For all films, we observe a smallincrease of the density with Tdep with the indication of asmall step in the temperature range of crystallization forfilms from the various precursors, Figure 5a. In Figure 5b,the change of density of amorphous films versus the post-deposition annealing temperature is plotted. The most sig-nificant densification is observed upon annealing at a tem-perature of 400 °C, which is below the crystallization tem-perature of most of the films and is independent of thedeposition temperature of the film as long as it is amor-phous. No significant further densification is observed withhigher temperature annealing in the range 425–600 °C. Thisbehavior is similar to earlier observations with HfO2

films,[12] and shows that this densification is not directly re-

lated to crystallization. The densification might be primar-ily related to the release of impurities like hydrocarbons,and time of flight secondary ion mass spectrometry (TOF-SIMS) can rationalize the observed trends, e.g., delayedcrystallization for hexane correlates with higher carboncontent at low temperatures.

The observed densities remain, however, below thetheoretical density for all polymorphs,[13] monoclinic(5.82 g cm–3), tetragonal (5.86 g cm–3), cubic (6.28 g cm–3)and orthorhombic (6.81 g cm–3) and below the density formonoclinic bulk ceramics (5.56 g cm–3).[14] Densities of allthe films except the [Zr(NEtMe)2(guanid)2]/toluene, whichdensified to 5.0 gm cm–3 upon annealing, were saturated to4.5 gm cm–3. Such low densities have been observed anddiscussed previously for CVD films of crystalline ZrO2.[15]

As these densities were 77 % and 86 % of the theoreticaldensity for the monoclinic structure, these lower densitieswere used to extract the correct thickness from the arealmass density deduced from X-ray fluorescence (XRF).

In spite of the great similarity in the behavior of the dif-ferent precursors, some characteristic differences can beseen. Most significant, the guanidinate-based precursoryields, for all Tdep, by far the highest density. Minor differ-ences are observed also for the alkoxide precursorsand the solvents. At low temperatures the films from[Zr(OiPr)2(tbaoac)2] in butyl acetate have a higher densitythan films from [Zr(OiPr)2(tbaoac)2] and [Zr(OtBu)2(t-baoac)2] in hexane. At high deposition temperatures thereis no significant difference in density.

2.3. Electrical Properties of ZrO2 on SiOx/Si(100)

Electrical properties of ZrO2 films in the MIS configura-tion were studied for crystalline as well as amorphous films.Annealing of the as-deposited films and annealing of thetop electrode were optimized to produce reproducible thinfilm behavior in terms of the electrical properties. Filmsconsidered here were annealed at 400 °C in a 50 %O2 and50 %N2 mixture for 5 min before the deposition of the Pttop electrode, and after Pt deposition, again annealed at400 °C for 20 min in forming gas. We start with examples ofthe capacitance–voltage (C–V) and current–voltage (I–V)curves for representative ZrO2 films of one of the precur-

sor/solvent combinations, and finally comparethe equivalent oxide thickness (EOT) and leak-age behavior of films from all the precursor/sol-vent combinations discussed in the precedingsections.

2.3.1. C–V Characteristics

C–V and loss tangent-voltage characteristicswere recorded by sweeping the DC bias (whichis superimposed on the AC signal having a volt-age level of 50 mV and frequency 100 kHz) from

Chem. Vap. Deposition 2007, 13, 98–104 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cvd-journal.de 101

20 30 40 50 60 70

(202)/

(220)

(002)/

(200)

(111)

82.1 nm

56.9 nm

34.1 nm13.1 nm

ZrO2/SiOx//Si

4.3nm

Inte

nsit

y (

arb

.un

its)

2θ (Deg.)

Fig. 4. XRD patterns of ZrO2 films having varying thicknesses deposited at600 °C from [Zr(OiPr)2(tbaoac)2] in n-butylacetate.

Table 2. Crystallization temperature (susceptor temperature) observed for as-deposited ZrO2

films on Si(100) substrates and for post-deposition annealing of amorphous ZrO2 films forvarious precursor/solvent combinations.

Precursor/solventAs-deposited films

Amorphous to crystalline(post-deposition annealing)

Thickness[nm]

Crystallizedat [ °C]

Thickness[nm]

Crystallizedat [ °C]

[Zr(OiPr)2(tbaoac)2]/butylacetate 10.0 450 13.3 450[Zr(OiPr)2(tbaoac)2]/hexane 12.0 550 10.1 500[Zr(OtBu)2(tbaoac)2]/hexane 18.4 550 10.6 500

[Zr(NEt2)2(dbml)2]/butylacetate 7.0 450 7.1 400[Zr(NEtMe)2(guanid)2]/toluene 14.7 400 7.9 400

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inversion to accumulation and back to inversion. Figure 6ashows the C–V of ZrO2 films of varying thicknesses pre-pared from the [Zr(NEt2)2(dbml)2]/butylacetate combina-tion. A systematic reduction in the saturation capacity,Cmax, with increasing thickness is observed. Effective di-electric constants of these films were 6.5, 10.7, and 15.3, re-spectively for films with thicknesses 2.9 nm, 5.9 nm, and11.9 nm. This linear increase in the effective dielectric con-stant is due to the additional capacitance of the interlayerbetween Si and the high-k film. The two contributions canbe separated assuming a two capacitor model with a stableinterfacial layer between Si and ZrO2. From a plot of thereciprocal capacitance, C, or EOT (EOT = (e0eSiO2A)/C;A= capacitor area) versus film thickness, the relative di-electric permittivity of the film can be calculated from theslope of the line, and the interfacial capacitance (capaci-tance of the interlayer between Si and high-k) is given bythe Y-offset (Fig. 7). Figure 6b shows the simultaneously-obtained loss tangent. A reduction of the value of the losstangent with increasing thickness is observed at accumula-

tion. The peak in this curve is close to the flat-band voltage,which was determined exactly by the Hauser and Ahmed

102 www.cvd-journal.de © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2007, 13, 98–104

as-dep 400 500 6002

3

4

5

6

(b)

ρtheor.

=5.817gm/cm3

(a) in- Butyl acetate

(a) in- Hexane

(b) in- Hexane

(c) in- Butyl acetate

(d) in- Toluene

Annealing temperature (ºC)

400 450 500 550 600 6502

3

4

5

6

(a)

ρtheor.

=5.817 gm/cm3

(a) in- Butyl acetate

(a) in- Hexane

(b) in- Hexane

(c) in- Butyl acetate

(d) in- Toluene

Den

sit

y (

gm

/cm

3)

Den

sit

y (

gm

/cm

3)

Susceptor temperature (°C)

Fig. 5. i) Density calculated from XRR spectra as a function of the susceptor temperature for all precursor/solvent combinations. ii) Thedensification of as-deposited amorphous films upon annealing in 50 % O2 and 50 %N2 mixture for 5 min: a) [Zr(OiPr)2(tbaoac)2],b) [Zr(OtBu)2(tbaoac)2], c) [Zr(NEt2)2(dbml)2], d) [Zr(NEtMe)2(guanid)2]. For precursor (a) the influence of the solvents (butylacetatevs. hexane) is also shown.

-2 -1 0 1 20.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

(a)

Cap

acit

an

ce (

µF

/cm

2)

Bias voltage (V)

tox

=2.94nm

tox

=5.88nm

tox

=11.93nm

simulation

-2 -1 0 1 2

0.00

0.05

0.10

0.15

0.20

0.25

0.30

(b)

tan

δ

Bias voltage (V)

tox

=2.94nm

tox

=5.88nm

tox

=11.93nm

Fig. 6. Thickness dependence of a) C-V, and b) loss tangent-voltage characteristics of ZrO2 films from [Zr(NEt2)2(dbml)2] in butyl acetate.

0 5 10 15 20 250

1

2

3

4

5

6

7

(a) in- Butyl acetate; εr=20

(b) in- Hexane; εr=24

(c) in- Hexane; εr=17

(d) in- Butyl acetate; εr=24

(e) in- Toluene; εr=24

EO

T (

nm

)

Thickness (nm)

Fig. 7. Comparison of the EOT vs. thickness of ZrO2 films from varioussolvent/precursor combinations. a) [Zr(OiPr)2(tbaoac)2], b) [Zr(OtBu)2-(tbaoac)2], c) [Zr(NEt2)2(dbml)2], d) [Zr(NEtMe)2(guanid)2]. For precursor(a) the influence of the solvents (butylacetate vs. hexane) is also shown.

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fit[16] included in Figure 6a. The shift in the flat-band volt-age towards the voltage axis with thickness is due to the in-crease of positive charges in the films as the thickness in-creases.

The average relative dielectric permittivities for filmsfrom the different precursors were obtained from the slopesshown in Figure 7 and values of 20, 24, 17, 24, and 24, respec-tively, were obtained for [Zr(OiPr)2(tbaoac)2]/butylacetate,[Zr(OiPr)2(tbaoac)2]/hexane, [Zr(OtBu)2(tbaoac)2]/hexane,[Zr(NEt2)2(dbml)2]/butylacetate, [Zr(NEtMe)2(guanid)2]/toluene combinations. Hence, high relative dielectric per-mittivities, which lie within the range of values reported forZrO2 (e = 22–25),[1,17,18] were obtained for all precursors ex-cept [Zr(OtBu)2(tbaoac)2]. The interfacial layer thicknessfrom the Y-intercept for the corresponding precursor/sol-vent combinations were 1.4 nm, 1.4 nm, 1.6 nm, 1.2 nm, and1.7 nm, and are reasonable values as no optimization was at-tempted here, and the native oxide was not removed.

2.3.2. I-V Characteristics

Figure 8a shows representative I–V behavior for films ofvarying thicknesses prepared from the [Zr(NEt2)2(dbml)2]/butylacetate combination. The leakage currents arestrongly reduced with increasing thickness. Typical valuesin the accumulation region, at an applied voltage of –1 V,were in the range 10–2 to 10–8 A cm–2. Finally, Figure 8bcompares the properties of the ZrO2 and the referenceSiO2 capacitors in terms of EOT and leakage current. Theleakage current densities of the ZrO2 films are consider-ably lower than those of SiO2,[19] but the EOT should be re-duced further while maintaining the same leakage currenttrend (see the dotted trend line). This implies a reductionof the thickness of the interfacial layer of SiOx from 2 nmto values < 1 nm. Possible lines for the progressing of theinterface are discussed in the literature.[18]

3. Summary and Conclusions

Four different mononuclear complexes of zirconium,the alkoxide-based precursors [Zr(OiPr)2(tbaoac)2] and[Zr(OtBu)2(tbaoac)2] and the amide-based precursors[Zr(NEt2)2(dbml)2] and [Zr(NEtMe)2(guanid)2] have beenevaluated for liquid injection MOCVD applications. De-positions were performed over a wide range of susceptortemperatures (400–700 °C) in order to grow amorphous aswell as crystalline films.

All precursors have some pros and cons and the finalchoice must depend on the details of the application. Forall precursor/solvent combinations the growth rate in-creased approximately exponentially along with the deposi-tion temperature, and saturated before decreasing again athigh deposition temperatures. From the growth prospectfor amorphous films at low temperatures (< 500 °C), theamide-based precursors, especially the guanidinate-basedprecursor, seem to show superior properties. On the otherhand [Zr(OtBu)2(tbaoac)2] seems to be the most stablecomplex and shows no decrease of the growth rate up to700 °C. In the case of [Zr(OiPr)2(tbaoac)2], there is also anindication that hexane becomes the better solvent at tem-peratures > 600 °C.

Crystallization temperature and film density, which seemcorrelated with the carbon content of the films, were de-pendent on the type of precursor and also on the solvent.Density of the as-deposited films increased with depositiontemperature but did not reach the bulk value. This studyclearly shows unique advantages of the new guanidinate-based precursor for depositions at low T in terms of surfaceroughness, density, and purity, as directly indicated by alow crystallization temperature. In addition we have indica-tions for solvent effects on the crystallization and densityof ZrO2 films and hence, in addition to solubility, other cri-teria must be considered in the selection of solvents for liq-

Chem. Vap. Deposition 2007, 13, 98–104 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cvd-journal.de 103

0 1 2 3

10-11

10-9

10-7

10-5

10-3

10-1

101

103

105

107

Reqd.trend line

(b)

SiO2

ZrO2

Cu

rren

t d

en

sit

y (

A/c

m2)

Cu

rren

t d

en

sit

y (

A/c

m2)

EOT (nm)

-2 -1 0 1 2

1E-11

1E-9

1E-7

1E-5

1E-3

0.1

(a)

2.93nm

5.88nm

11.93nm

Applied voltage (V)

Fig. 8. a) I-V characteristics of Pt/ZrO2/Si MOS capacitors having various insulator thicknesses for films from [Zr(NEt2)2(dbml)2] in butyl acetate.b) Comparison of the capacitor properties of ZrO2 films from all the precursors with conventional SiO2 films [14].

Full Paper

uid injection MOCVD. For depositions at high T (> 550 °C)the combination of [Zr(OiPr)2(tbaoac)2] with hexane mightbe advantageous and the substitution of the alkoxide li-gands from O-iPr to O-tBu might have advantages forT > 650 °C.

Without optimizing the interface between Si and ZrO2

good electrical bulk properties of the films, relative di-electric permittivity, and leakage current, were obtained.Hence, the engineered alkoxide- and amide-based precur-sors are promising candidates for applications in informa-tion technology.

4. Experimental

ZrO2 films were deposited in an AIXTRON 2600G3 Planetary Reactor,which can handle five 6 inch wafers simultaneously. Being characterized bya central gas inlet makes this reactor a radial flow system [11]. Growth tem-peratures refer to the temperature measured at the graphite susceptor,which may be 20 to 50 °C higher than the surface temperature of the sub-strates.

The areal mass density of the deposited Zr was routinely determined bywavelength dispersive XRF, using calibration standards deposited by chemi-cal solution deposition. In order to deduce the film thickness from the XRFdata, the bulk density for ZrO2 of 5.8 g cm–3 was used. Hence, this thicknesscorresponds to the equivalent thickness of a ZrO2 layer with the bulk den-sity and presents a lower limit for the actual film thickness, as the density ofthe film is generally lower than the bulk value, therefore additional directmeasurements of thickness and film density were performed by XRR.Thickness is calculated from the spacing between interference fringes, anddensity is related to the critical angle for total reflection, hc [20].

The phase determination of the films was carried out on a Philips MRDX-ray diffractometer, using Cu Ka radiation and a diffracted beam mono-chromator. Generally, glancing incidence geometry with an angle of inci-dence of 20 was used. Surface morphology of the films was studied withAFM using a SIS pico-station. The carbon content was investigated withTOF-SIMS using a TOF-SIMS-IV instrument from ION-TOF. Sputteringwas performed with 2 keV Cs ions; a Bi ion beam and negative polarity wasused for analysis. As, for such ‘insulating’ films, ionization probabilities varystrongly with local conductivity, an absolute calibration seems problematic,only the relative carbon content of the films is discussed.

For electrical measurements, Pt electrodes were sputter deposited on topof the ZrO2 films and patterned, with a pad size of 0.0491 mm2. The result-ing film stack was Pt/ZrO2/p-Si/Ga0.85In0.15. C–V characteristics were ob-tained with a HP #4284 LCR meter at 100 kHz. I–V curves, were measuredusing a Burster 4462 voltage generator and a Keithley 617 electrometer.

Received: April 4, 2006Revised: July 18, 2006

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