Atomic Layer Deposition of HfO2 Thin Films Employing a Heteroleptic Hafnium Precursor

DOI: 10.1002/cvde.201106934

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Atomic Layer Deposition of HfO2 Thin Films Employinga Heteroleptic Hafnium Precursor**

By Ke Xu, Andrian P. Milanov, Harish Parala, Christian Wenger, Canan Baristiran-Kaynak, Kaoutar Lakribssi,

Teodor Toader, Claudia Bock, Detlef Rogalla, Hans-Werner Becker, Ulrich Kunze, and Anjana Devi*

The application of a heteroleptic hafnium amide-guanidinate precursor for the deposition of HfO2 thin films via a water-assisted

atomic layer deposition (ALD) process is demonstrated for the first time. HfO2 films are grown in the temperature range 100–

300 8C using the compound [Hf(NMe2)2(NMe2-Guan)2] (1). This compound shows self-limiting ALD-type growth charac-

teristics with growth rates of the order of 1.0–1.2 A per cycle in the temperature range 100–225 8C. The saturation behavior and alinear dependence on film thickness as a function of number of cycles are verified at various temperatures within the ALD

window. The as-deposited HfO2 films are characterized by atomic force microscopy (AFM), scanning electron microscopy

(SEM), Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS), and electrical measurements.

For a direct comparison of the precursor performance with that of the parent alkyl amide [Hf(NMe2)4] (2), ALD experiments

are also performed employing compound 2 under similar process conditions, and in this case no typical ALD characteristics are

observed.

Keywords: ALD, HfO2, Precursors, Thin Films, Composition

1. Introduction

HfO2 is regarded as one of the promising materials for

gate oxide applications in complementary metal oxide

semiconductor (CMOS)-based devices because of its

dielectric constant (�20–25), large band gaps (�5.5 eV)

and band off-sets relative to Si, as well as good thermo-

dynamic stability on Si wafers.[1–4] The ALD process takes

advantage of surface saturation reactions to deposit

extremely smooth, dense, and highly conformal films

through a process that is relatively insensitive to fluctuations

in process temperature and reactant flux.[5] Over the last few

years the semiconductor industry has been adopting the

ALD technique for growing thin layers because the benefits

of ALD enable scaling and improved performance. The

[*] Prof. A. Devi, K. Xu, Dr. A. P. Milanov, Dr. H. ParalaInorganic Materials Chemistry,Ruhr-University Bochum 44801 Bochum (Germany)E-mail: [email protected]

Dr. C. Wenger, C. Baristiran-Kaynak, K. LakribssiIm Technologiepark 25, 15236 Frankfurt (Oder) (Germany)

T. Toader, Dr. C. Bock, Prof. U. KunzeElectronic Materials and Nanolectronics, Ruhr-University Bochum44801 Bochum (Germany)

Dr. D. Rogalla, Dr. H.-W. BeckerDynamitron-Tandem-Laboratory (DTL) of RUBION,Ruhr-University Bochum 44801 Bochum (Germany)

[**] The authors from RUB gratefully acknowledge the German ResearchFoundation (DFG) for funding this project (DFG-DE-790-9-1) as wellas Prof. R. A. Fischer for his continuous support. KX expresses hisappreciation to the RUB-Research School for granting a PhD researchfellowship.

Chem. Vap. Deposition 2012, 18, 27–35 � 2012 WILEY-VCH Verlag Gm

intrinsic parameter for a successful ALD process is the

precursor employed. The precursors for ALD of HfO2 have

been mainly chosen from either the chloride[6] or amide[7,8]

class of compounds. The amides are particularly interesting

because of their non-corrosive nature when compared to the

chlorides. Although the amides are volatile and reactive for

ALD applications, the limited thermal stability of the parent

amides can affect the film growth mechanism during an

ALD process.[9,10]

Among the different Hf amides, the tetrakis-ethylmethy-

lamido-hafnium (TEMAH) complex has been investigated

in detail with various co-precursors such as H2O or O3.[7–11]

There are various reports on the decomposition temperature

of this precursor during an ALD process, and it was

surmised that the decomposition depends on the reactor

design and the process conditions.[7,8,11] A recent report

highlighted that parasitic growth is associated with ALD

using [TEMAH]/H2O owing to the limited thermal stability

of the precursor.[9] Surprisingly, the ALD characteristics of

the other two closely related amides, namely tetrakis-

diethylamido-hafnium (TEAH) or tetrakis-dimethylamido-

hafnium (TMAH), had not, until recently, been reported in

detail. TEAH was used in combination with the analogous

Zr (TEAZ) compound to deposit HfZrO4 films.[12]

TEMAH, TMAH, and TEAH were studied recently using

a discrete feeding method to obtain improved growth rates

during the ALD process using O3.[10] Although the typical

ALD characteristics of HfO2 growth using TEAH and

TMAH with H2O have not been discussed in detail, the

functional properties of HfO2 films obtained from these two

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precursors have been reported earlier.[13,14] Since the

hafnium amides are largely promising (high growth rates)

for ALD applications but do have certain drawbacks in

terms of limited thermal stability and handling (very

reactive), efforts have been made to improve their thermal

stability. In the past, we have modified the physico-chemical

properties of hafnium amides in an attempt to improve the

thermal stability of the parent hafnium amides.[15,16] This

was achieved by introducing chelating ligands such as

guanidinates and malonates into the hafnium metal ligand

sphere with the specific intention of enhancing the thermal

stability. This concept was attempted for all the three parent

hafnium amides [Hf(NR2)4] (R¼Me, Et, EtMe), and we

were successful in obtaining six coordinated Hf complexes

which showed improved thermal properties.[15–17] The

application of these so called ‘‘designer’’ precursors was

very successful in state-of-the-art metal-organic (MO)CVD

reactors.[18] In particular, the mixed Hf-amide-guanidinate

class of precursors showed promise for the growth of high

quality HfO2 thin films by MOCVD exhibiting promising

electrical properties.[19] Using a similar strategy, mixed

Hf-amide-cyclopentadienyl precursors were developed,

and ALD of HfO2 was demonstrated using ozone as the

oxidant.[20]

The focus of this work is to evaluate the modified hafnium

amide precursor, namely Hf-amide-guanidinate (1), for the

ALD of HfO2 in combination with water. To make a direct

comparison of the ALD characteristics using compound 1

with respect to the parent hafnium amide compound 2, HfO2

was grown using compound 2 under similar process

conditions in the same reactor (ASM-F-120). The ALD

process based on compound 1 led to the growth of HfO2 at

low temperatures, which can be advantageous for applica-

tions where low fabrication temperatures are necessary.

Moreover, the use of water instead of ozone to assist the

ALD process is an advantage in reducing the interfacial

oxide growth. The ALD characteristics of both compounds

were studied, and the films obtained from compound 1

were subjected to detailed analysis employing AFM, SEM,

RBS, and XPS. Preliminary electrical characterization was

performed on MOS capacitor structures using capacitance/

voltage (C-V) and current/voltage (I-V) measurements.

2. Results and Discussion

The strategy of using heteroleptic compounds for ALD

applications can be generally explained as follows. The

thermal and chemical properties of heteroleptic compounds

can be influenced or tuned by employing various ligand

systems or varying the terminal groups in the ligand

periphery. The metal center can be coordinatively saturated

and the reactivity of the compound can be significantly

altered. In our previous study, the Hf-amide-guanidinate

complex was reported to be a suitable precursor for ALD

applications as it possesses enhanced thermal stability, apart

28 www.cvd-journal.de � 2012 WILEY-VCH Verlag GmbH

from being volatile at moderate temperatures (100 8C). Thecompound was relatively less sensitive to air and moisture

compared to the parent alkyl amide. The presence of metal-

nitrogen bonding contributes to the reactivity of the

complex. The higher thermal stability of the complexes

was verified using thermal analysis and nuclear magnetic

resonance (NMR) decomposition studies.[17] Accordingly,

this precursor is stable for several days (9 days) even when it

is maintained at temperatures as high as 140 8C, which could

be a typical deposition temperature in an ALD process. On

the other hand, the precursor reactivity is not reduced, as

seen from the reactivity towards water and high growth rates

obtained in anALDprocess. There is thus an optimal balance

between reactivity and thermal stability which are the

prerequisites for anALD precursor. In general, the improved

thermal stability of compound 1 compared to compound 2,

and the necessary reactivity the Hf-amide-guanidinate

possesses, are the most important factors in using this as

an ALD precursor for HfO2 thin film deposition.

2.1. Precursor Evaluation

The synthesis of compound 1 was scaled up to larger

batches (10–20 g). The thermal analysis of the two

compounds was performed, and the thermogravimetry

(TG) curves, recorded as a function of temperature, are

shown in Figure 1a. The onset of volatilization of the parent

amide, compound 2, is around 75 8C, while the onset of

volatilization is shifted to higher temperatures for com-

pound 1. It can clearly be seen, however, from the TG

measurements that compound 2 decomposes at tempera-

tures as low as 150 8C, while the decomposition of precursor

1 occurs at approximately 250 8C.A very low residual mass is

observed in the case of compound 1 (�4%), but in the case

of compound 2 it was higher (�12%). The isothermal studies

(Fig. 1b) show that compound 1 can deliver a constant mass

transport during the deposition process, even at tempera-

tures as high as 160 8C. No decomposition is observed during

the sublimation process. Hence, from the thermal analysis

data, it can be inferred that compound 1 is volatile and

thermally stable up to 250 8C. Temperature-dependent

NMR decomposition studies revealed that compound 1

shows extraordinary thermal stability.[17] The promising

attributes of this compound, which include good volatility,

high reactivity, and very good thermal stability, make

guanidinate-based Hf-amide precursors appealing for ALD

of HfO2 thin films.

2.2. Deposition of HfO2 Thin Films

2.2.1. ALD Growth Characteristics

A distinctive feature of a true ALD process is the ALD

window where the growth rate is minimally affected by the

deposition temperature. Although anALDwindow is not an

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Fig. 1. a) TG curves for compound 1 (black) and compound 2 (red).

b) Isothermal TGA curves for compound 1 under atmospheric pressure

under constant N2 flow (300mLmin�1).

Fig. 2. The growth per cycle of HfO2 films deposited from a) compound 1 and

b) compound 2, as functions of deposition temperature.

absolute necessity for the ALD-type growth mode, it is

interesting to know the temperature up to which the

precursor is thermally stable under the adopted process

conditions, and the temperature window (i.e., the low and

high end temperatures within which it can be operated with

reproducibility). Therefore, the first set of experiments using

compound 1 in combination with water was to check the

temperature dependence on growth rates, and these data are

presented in Figure 2a. Under the adopted process

conditions in this study, HfO2 film growth was observed

at temperatures as low as 100 8C, and a constant growth rate

of 1.0–1.2 A per cycle were obtained up to approximately

225 8C. The growth rates are slightly higher than reported

for mixed amide-cyclopentadienyl precursors with ozone as

the oxidant.[20] In comparison with other HfO2 ALD

processes, the growth per cycle of compound 1/H2O in

the ALD window is 1.5 times higher than in the case of

amide based precursor (e.g., compound 2/H2O ca. 0.8 A

per cycle at 250 8C) and two times higher than HfCl4 (0.5 A

per cycle at 300 8C).[7,21] The higher growth per cycle

observed in the case of compound 1 can probably be


explained by the presence of highly reactive dimethyl-amide

groups. Beyond 250 8C, a surge in the growth rate was

observed (1.7–1.8 A per cycle) which indicates a possible

CVD contribution resulting from precursor decomposition.

The temperature value for the onset of precursor decom-

position during the film depositionmatches the one obtained

from TG analysis for this precursor (Fig. 1a). If the

deposition temperature was further increased to 275 8Cand above, film growth per cycle decreased, and no film

growth could be observed beyond 350 8C. This is probablydue to a predominant premature thermal decomposition of

the precursor.

In order to verify the thermal decomposition of this

precursor in the ALD reactor, a series of separate

experiments were carried out where compound 1 was

pulsed 400–600 times, with a pulse length of 2 s, without

introducing water. No film growth could be observed when

the deposition temperature was lower than 200 8C. Above

225 8C, layers with unknown composition were formed on

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Fig. 3. HfO2 growth per cycle as a function of pulse time. a) At 150 and 200 8Cfor compound 1, and b) at 200 8C for compound 2.

the Si substrates, indicating thermal decomposition of this

precursor. This phenomenon was consistent with the

thermal characteristics of this precursor which have been

reported earlier, namely, that compound 1 starts to

decompose at temperatures of approximately 250 8C.[15]

Since we were interested in comparing the ALD

performance of compound 1 with the parent amide

(compound 2), similar deposition experiments were carried

out under identical experimental conditions, again using

water as the reactant. Surprisingly for the parent amide, very

high growth rates were obtained (more than double) and the

growth rate steadily increased from 125 to 225 8C, beyondwhich it decreased rapidly. The growth per cycle of HfO2

thin films deposited with compound 2/H2O was ca. 2.3 A per

cycle at 125 8C, and it increased to ca. 2.7–2.9 A per cycle in

the temperature range of 175–225 8C. Beyond 250 8C, thegrowth rates decreased, and at 300 8C, the growth per cycle

was reduced to 2.5 A per cycle, which can probably be

explained by the enhanced gas-phase decomposition of

compound 2. There was no clearly defined ALD window in

the temperature range investigated for compound 2, in

contrast to a defined ALD window in the case of compound

1. These results justify our observation of the thermal

analysis which shows that the parent hafnium amide has a

limited thermal stability. It should be noted that there are

discrepancies in the values of the ALD temperature window

and decomposition temperature in the reported ALD

processes using the same class of precursors. Particularly

in the case of the most frequently used [Hf(NEtMe)4]

precursors, it was reported that this compound tends to

decompose at 300 8C or even below.[9] As discussed in the

Introduction, the decomposition depends on the reactor

design and process conditions.[7–9,11] This prompted us to

carry out a comparative study using the compounds 1 and 2

with the same reactor and under identical process conditions

to avoid any ambiguity.

Apart from the ALD window, the surface-controlled self-

terminated ALD growth behavior, and the linear depen-

dence of film thickness on the number of ALD growth

cycles, characterize a typical ALD process. In this context,

we investigated the influence of precursor pulse length on

the growth per cycle, and the data are depicted in Figure 3a

(for compound 1). A series of experiments were carried out

at various substrate temperatures, and the pulse lengths

were varied in steps of 0.5 s. Irrespective of the deposition

temperature, a saturative growth mode was attained when

the pulse length was increased to 1.5 s, and the growth rate

was of the order of 1.1 A per cycle. No significant increase in

the growth per cycle was observed when the pulse length was

increased beyond 1.5 s. In the self-limiting region, a growth

rate in the range 1.1–1.3 A per cycle was obtained for the two

different temperatures investigated (150 and 200 8C). Theuniformity of the films on 2-inch Si wafers was not affected,

even when the pulsing timewas increased to 10 s. The studies

on the film thickness variation as a function of ALD growth

cycles using compound 1 revealed a linear dependence


(Fig. 4a), which confirms the other characteristic feature of

an ALD process.

On the contrary, no self-limiting behavior was observed

for compound 2 at the various temperatures investigated,

even when the precursor length was increased to 5 s

(Fig. 3b). The growth per cycle with increasing pulse length

was extremely high (ca. 6 A per cycle) which is 3–4 times

higher than the commonALDprocess for HfO2. A plausible

reason for this is the high ratio of the CVD-like contribution

under the adopted process conditions. This argument can be

supported with the data reported using a similar precursor

[Hf(NEtMe)4].[9] In this report, a thorough study using this

precursor was performed and it was found that parasitic

growth behavior was observed at higher temperatures

(CVD-like contribution) due to the insufficient thermal

stability of the precursor. In our case, this phenomenon is

evenmore evident, which is probably due to the lower thermal

stability of compound 2 compared to [Hf(NEtMe)4]. To justify

this claim, in-situ measurements need to be performed in


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Fig. 4. The dependence of HfO2 film thickness on the number of deposition

cycles. a) At 150 and 200 8C for compound 1, and b) at 200 8C for compound 2.

order to investigate the growth mechanism (which is beyond

the scope of this work).

The absence of an ALD-type growth behavior with

compound 2was further verified when linear dependence on

film thickness as a function of ALD cycles was not observed

(Fig. 4b). Taking these experimental observations into

consideration, compound 2 does not seem to function as an

ALD precursor using the set of process conditions and the

reactor that we have employed. On the other hand, true

water-assisted, low-temperature ALD behavior was

observed using the modified amide precursor (compound

1). Another point to be noted is that the sticking effect of

hafnium alkyl amides has been discussed in the literature,

and a time scale of 100 s is required in order to ensure that

excess precursor is purged away.[13] In the case of compound

1, the steric bulkiness of our precursor perhaps reduces the

sticking effect and it is possible to achieve a precursor

monolayer within 20–30 s, if a precursor pulse length of 5 s is


employed. To summarize, under similar process conditions,

ALD growth characteristics can be achieved with compound

1 but not for the parent Hf-amide (compound 2). Based on

these observations it could be argued that precursor 2 has a

reduced sticking effect in comparison to compound 1,

however to verify this hypothesis further detailed studies are

warranted. Since compound 2 does not typically exhibit

ALD characteristics and the film properties using compound

2 have been reported earlier,[14] our focus was on

characterizing films obtained only from compound 1 and

the results are discussed in the following section.

2.2.2. HfO2 Film Characterization

The as-deposited HfO2 films in the temperature range

100–250 8C using compound 1 had a thickness of the order of

10–50 nm. Grazing incidence X-ray diffraction (GI-XRD)

was performed to check the crystallinity of the films

(2u¼ 20–608), and the films were found to be amorphous

(Supplementary Information, Fig. SI-1). The morphology of

the films was analyzed by SEM measurements, where a

featureless surface was observed (Fig. SI-2) for films grown

in the range 100–250 8C. The surface roughness of the HfO2

thin films deposited at 125 8C, 150 8C, 200 8C, and 250 8Cwas

analyzed by AFM (Fig. 5). The films were found to be very

smooth and the rms roughness values ranged from 0.15 nm

(125 8C) to 0.34 nm (150 8C). The rms/thickness ratios varied

from 0.6% (200 8C) to 1.3% (250 8C). No significant

variation of the roughness with temperature was noticed.

These roughness values are lower than those reported in the

literature in terms of rms/thickness ratio, such as 5.98% in

the case of [Hf(NEtMe)4] at 200 8C with a comparable film

thickness.[7]

The composition of the films was analyzed by RBS, and

the presence of Hf and O in the films on Si(100) at various

deposition temperatures was confirmed by these measure-

ments (Fig. 6). There was no evidence of any contamination

by the lighter elements (N and C) within the detection limit

of RBS (� 10%). The HfO2 films grown in the ALD

temperature window 100–225 8C were stoichiometric. The

O/Hf ratio was 2.0 (� 0.05) in the temperature range 125–

225 8C, and it increased slightly to 2.2 (� 0.05) at 250 8C.XPS

depth profile analysis was performed in order to get the in-

depth composition of the films. Figures 7a and 7b show the

XPS depth profiles of HfO2 on Si substrates deposited at

125 8C and 250 8C, respectively. The profiles have been

obtained with the sputtering step of 1min. The surface

carbon contamination is removed after 1min Arþ beam

sputtering and the residual carbon was very low (around

2%) throughout the HfO2 films in both cases. The film

composition obtained after 1min sputtering was nearly

stoichiometric, however further sputtering on the samples

resulted in non-stoichiometric compositions. A similar

phenomenon has been reported in the literature where

Arþ sputtering on HfO2 film causes preferential sputtering

of O with respect to Hf and thus, non-stoichiometric

composition is obtained by depth profiling.[22]


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Fig. 5. AFM images of ca. 15 nm thick HfO2 thin film deposited using compound 1/H2O process at 125 8C, 150 8C, 200 8C, and 250 8C.

In order to clarify the chemical states of elements on the

surface of the films, the XPS measurements were carried out

and the deconvoluted spectra of Hf 4f, O 1s, and C 1s are

shown in Figures 8a–c, respectively. The analysis of Hf 4f

spectra reveals the presence of a double peak structure with

spin orbit splitting 1.6 eV, and the area ratio between

components of 1.3 in both samples. The peaks observed at

16.9 eV and 18.5 eV correspond to Hf 4f5/2 and Hf 4f7/2respectively, and are related to Hf-O bonding in HfO2.

[23–25]

The small peak obtained for both films at around 16 eV

might be attributed to a Hf-C bond on the surface of the

films.[26]

Figure 8b demonstrates the O 1s core level spectra

obtained from both films. Each spectrum was deconvoluted


into three separate peaks corresponding to different

chemical states. The small peaks at around 532 eV and

529 eV might be attributed to C�O, C¼O bonds in

carbonate groups, and surface absorbed oxygen species

(O�, O�2, or O�2 ), respectively.[27,28] The main peak

centered at 530.4 eV is attributed to the Hf�O bond in

HfO2.[23] The C 1s core level XPS spectra (Fig. 8c) were

composed of three separate component peaks at binding

energies of around 289, 284.8, and 282,5 eV which can be

ascribed to C�O,C�C,[27] andHf�C,[26] respectively. In the

literature, it has been shown that HfO2 can react with

atmospheric CO2 to form carbonates.[29] The shift of

�0.8 eV observed at the corresponding region of C�O

between the samples might be due to the different


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Fig. 6. RBS spectra of HfO2 films deposited on Si(100) at 125 8C, 150 8C,200 8C, and 250 8C using compound 1/H2O process.

Fig. 7. XPS depth profile of HfO2 films grown on Si substrate at a) 125 8C and

b) 250 8C.

composition of this component on the surface of the

films.

The preliminary electrical characterization was per-

formed on 40 nm thick HfO2 layers, deposited on silicon

substrates with gold as the top electrode material. The

effective dielectric constant of HfO2 as a function of the

deposition temperature was extracted from high-frequency

C-V measurements in accumulation, as shown in Figure 9a.

The effective k-value increases with deposition temperature

from 8 (at 125 8C) to 18 (at 250 8C), which is close to the

theoretical value (�22) of amorphous HfO2.[30] The

deposition temperature also affects the leakage current

density in accumulation, as shown in Figure 9b. As a

tendency, the leakage current density decreases with

increasing deposition temperature. The amorphous HfO2

films, deposited at 250 8C, exhibit the lowest leakage current

Fig. 8. Deconvolution of core level XPS spectra of HfO2 grown at 125 8C and 250 8C; a) Hf 4f, b) O 1s, c) C 1s.

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Fig. 9. a) Effective dielectric constant and b) leakage current density, as

functions of electric field of HfO2 films deposited at various temperatures.

density of 5� 10�8 A cm�2 at �1V. The preliminary

electrical results are encouraging, and a more detailed study

of the effect of film thickness on the electrical properties is

warranted in order to compare the data with published

results using other amide-based precursors.

From our earlier studies we have shown that using the

guanidinate ligand system, Gd2O3 films of high quality were

grown by water-assisted ALD in the same temperature

range.[31] The overlapping ALD window for HfO2 and

Gd2O3 should enable the development of a truly water-

based ALD process for rare-earth (e.g., Gd, Dy) doped

HfO2 using the similar class of precursors.[32] This could be

an added advantage for the optimization of theALDprocess

to grow high dielectric constant HfO2 thin films in the cubic

or tetragonal phase. Research efforts in this direction are

currently underway.

3. Summary and Conclusions

A low temperature, water-assistedALD process for HfO2

was developed using the modified heteroleptic hafnium


amide precursor (1). Typical ALD characteristics were

verified in terms of a well-defined ALD window, saturation

behavior, and linear dependence of the growth rate on the

number of cycles. The growth rates are comparable to the

ones reported for TEMAH. On the contrary, the use of the

parent amide TMAH under the same process conditions did

not show any true ALD characteristics. The XRD results

obtained for films grown from compound 1/H2O indicate

that the as-deposited films in the ALD window are

amorphous. The films are stoichiometric, as revealed from

RBS measurements, and nearly free of carbon, as inferred

from XPS depth profile analysis. From the results obtained

in this study, we conclude that the thermal stability of this

precursor is higher than the corresponding parent amide.

The presence of the Hf-N bond in the precursor molecule

contributes to the reactivity, and saturation of the hafnium

metal center with the chelating guanidinate ligands

enhances the thermal stability. The preliminary electrical

results are encouraging for the use of these HfO2 layers as

high-k oxides.

4. Experimental

4.1. Precursor Synthesis and Analysis: The precursors employed,namely bis(dimethylamido)-bis(N,N(-diisopropyl-2-dimethylamido-guanidi-nato)hafnium(IV) (compound 1) and compound 2, were synthesizedfollowing literature procedures [15]. All reactions were performed witha conventional vacuum/argon line using standard Schlenk techniques.Compound 1 is a solid and was purified by sublimation in vacuum (ca.120 8C/0.05mbar) after recrystallization from hexane, while compound 2 waspurified via vacuum distillation (ca. 100 8C/0.05mbar). The purity of theprecursors was further investigated using NMR and elemental analysisrecorded on a Bruker Advance DPX spectrometer and CHNSO Vario ELanalyzer, respectively. Thermogravimetric analysis (TGA) data wereobtained on a Seiko TG/DTA 6300S11 instrument. The heating rate was5 8Cmin�1 and a nitrogen flow of 300mLmin�1 was used. The preparation ofsamples for chemical analysis and ALD experiments was carried out in anargon-filled glove box (MBraun).

4.2. HfO2 Thin Film Deposition: Thin HfO2 films were deposited in acommercial, flow-type, hot-wall, ALD reactor (F-120 -ASM MicrochemistryLtd., Finland) on 2 inch p-type Si(100) substrates (SI-MAT) using the twodifferent precursors via compounds 1 and 2. The substrates wereultrasonically cleaned in acetone and ethanol, rinsed with deionized water(Millipore Water Purification System), and dried under argon stream. Thenative oxide layer was not removed prior to deposition. The Hf precursorswere handled in a glove box, inserted into the reactor, and evaporated froman open crucible maintained at 140 8C for compound 1 and 50 8C forcompound 2. About 200mg of the precursor was used for each deposition.De-ionized water (Millipore Water Purification System) evaporated from astainless steel container maintained at 25 8C was employed as the oxygensource. High-purity nitrogen (99.9999%) was used as both carrier and purginggas. The reactor pressure during the deposition was around 1–3mbar. TheHfO2 growth was studied in the temperature range of 100–300 8C forcompound 1 and 125–300 8C for compound 2 using the following standardpulsing sequence (ALD growth cycle): 0.5–10 s of metal precursor pulse,followed by 15–40 s of nitrogen purge, 1 s of water pulse, and finally 10 s ofnitrogen purge. The optimized conditions used were: 2 s precursor purge with20 s purge time for compound 2. The high purge time was to ensure that theexcess precursor was flushed out.

4.3. Thin Film Characterization: Crystallinity of the films wasinvestigated by carrying out grazing incidence X-ray diffraction (GI-XRD)analysis in a Bruker AXS D8 Advance diffractometer, using Cu Ka radiation(1.5418 A). The surface morphology was studied by AFM and SEM. AFMmeasurements in the tapping mode were performed using a NanoscopeMultimode V AFM (Digital Instruments), while SEM experiments wereperformed employing an instrument from LEO Zeiss. The film thickness was


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determined by spectroscopic ellipsometry (SE) measurements performed ona J. A.WoollamCo. ellipsometer from 283.5–751.8 nm using an incident angleof 75-. Film composition and stoichiometry were determined by XPS,including XPS depth profile and RBS. XPS studies were carried out using aPerkin Elmer PHI 5600 ci ESCA system with a residual gas pressure betterthan 1T 10�8 Pa. Monochromatic Al Ka radiation was used to excite thephoto-electrons at the take-off angle of 45- from the sample, and the passenergy of the spectrometer was set to 29.35 eV. For XPS depth profiling, anArR ion beam with energy of 2 keV was used. Calibration of the XPS spectrawas done with the surface carbon by setting C 1s as 284.8 eV. RBSmeasurements using 2.0MeV 4HeR ions were performed using an instrumentfrom the Dynamitron Tandem Laboratory (DTL) in Bochum. A beamintensity of about 20–40 nA incident to the sample at a tilt angle of 7- wasused. The backscattered particles were measured at an angle of 160- by a Sidetector with a resolution of 16 keV. The stoichiometry of the films wascalculated with the program RBX [33] by using the stopping powers of theprogram SRIM [34]. In order to characterize the electrical properties of theHfO2 films, electrical measurements (C-V/I-V) were performed on metaloxide semiconductor (MOS) test capacitors formed by evaporation of Au topelectrodes (contact area¼ 2T 10�3 cm2) on the HfO2/Si structures. Al wasused as backside contact on Si. C-V measurements of MOS capacitors wereperformed at 100 kHz in the range from �3V to R3V.

Received: April 14, 2011

Revised: July 08, 2011

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Atomic Layer Deposition of HfO2 Thin Films Employing a Heteroleptic Hafnium Precursor

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