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Journal of Crystal Growth 263 (2004) 442–446

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82-54-279-

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0022-0248/

doi:10.101

Characterization of hafnium silicate thin films grown byMOCVD using a new combination of precursors

Jaehyun Kim, Kijung Yong*

Surface Chemistry Laboratory of Electronic Materials, Electrical and Computer Engineering Division,

Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hoyja-dong,

Nam-gu, Pohang, Kyungbuk 790-784, South Korea

Received 2 October 2003; accepted 9 December 2003

Communicated by G.B. Stringfellow

Abstract

Hafnium silicate [(HfO2)X(SiO2)1�X] films were deposited by metalorganic chemical vapor deposition using a new

combination of precursors: tetrakis-diethylamido-hafnium [Hf(NEt2)4] and tetra-n-butyl-orthosilicate [Si(OnBu)4]. An

atomically flat interface of silicate/silicon was observed with no interfacial silicon oxide layers. The impurity

concentrations in grown films were less than 0.1 at% (below detection limits). Hafnium silicate films were amorphous

up to 800�C. Above 900�C, phase separation of the films occurred into crystalline HfO2 and amorphous Si-rich silicate

phases. Dielectric constant (k) of the Hf-silicate films was about 8. Hysteresis in capacitance–voltage (C2V )

measurements was less than 0.1V.

r 2003 Elsevier B.V. All rights reserved.

PACS: 84.37; 52.75.R; 81.15.G; 77.55,68.55.L; 77.84.B

Keywords: A3. Metalorganic chemical vapor deposition; B1. Oxides; B2. Dielectric materials

1. Introduction

In a sub-0.1 mm complementary metal-oxide-semiconductor technology, sub-1.5 nm SiO2 layerswill be required but at these thicknesses a directtunneling leakage current through SiO2 becomesunacceptably high [1]. Therefore, alternative ma-terials with a permittivity (k) higher than that ofSiO2 (3.9) are needed to be able to use thicker gate

onding author. Tel.: 011-82-54-279-2278; fax: 011-

8298.

address: kyong@postech.ac.kr (K. Yong).

$ - see front matter r 2003 Elsevier B.V. All rights reserve

6/j.jcrysgro.2003.12.009

dielectrics in achieving the required capacitancewithout tunneling currents [2]. Among high-kmaterials, HfO2 and ZrO2 are attractive candidatesdue to high stability with the Si substrate, based onGibbs free energy analysis under equilibriumconditions [3]. In practice, however, during filmdeposition or high-temperature annealing process,atomic oxygen (O) very fast diffuses through thesefilms either in a vacancy sub-lattice or grainboundary so that interfacial layers are formed atHfO2/Si and ZrO2/Si interfaces [4]. The interfacialsilicon oxide layer limits the maximum achievablecapacitance of the gate stack, or equivalently, the

d.

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J. Kim, K. Yong / Journal of Crystal Growth 263 (2004) 442–446 443

minimum achievable teq value, where teq is anelectrical thickness equivalent for pure SiO2 [5].As a gate oxide becomes thinner, the interfaces

between the high-k material and Si substrate aswell as poly-Si electrode have shown to play a keyrole in device performance. For these reasons Hf-or Zr-silicates are the most promising dielectricmaterials in direct contact with Si [6]. Silicate filmscan avoid the formation of low-k interfacial oxidelayers due to a high resistivity to oxygen diffusion[5,7]. Also, the incorporation of SiO2 into HfO2and ZrO2 films helps stabilizing an amorphousstructure during high-temperature annealing withenhanced interface carrier mobility [2,7,8]. Theoverall permittivity of silicate film is inevitablylower than that of the pure metal oxide, but thistradeoff can be very adequate for the improvedstability.Among various methods to grow silicate films,

chemical vapor deposition (CVD) and atomiclayer deposition are generally preferred for depos-iting ultra-thin films because they ensure good stepcoverage, up-scalability, and more controllableprocessing [2,9,10]. Most previous growth worksof Hf-silicates used HfCl4 as Hf precursor andvarious types of silicon precursors [2,11,12]. Incombination with these metal sources, severaloxygen sources are used to deposit silicate filmsincluding water, ozone, and alkoxides[2,6,9,11,12]. Metal-chloride precursor has goodthermal stability and high vapor pressure, but itinduces chlorine contaminations [2,6,9,11,12].Also, in general, metal-chloride source materialsrequire high growth temperatures.In this paper, we report the deposition of

Hf-silicate thin films by metalorganic chemicalvapor deposition (MOCVD) using a new combi-nation of precursors: tetrakis-diethylamido-haf-nium [Hf(NEt2)4] and tetra-n-butyl-orthosilicate[Si(OnBu)4]. We chose Hf(NEt2)4 as a Hf precursorto minimize carbon and chlorine contaminationsin grown films. Since Hf(NEt2)4 has Hf–N bondsinstead of Hf–Cl, it is expected to produce lesscarbon and chlorine contaminations in grownfilms [13]. Hf(NEt2)4 is liquid at room temperatureand has moderate vapor pressure, meaning that itis a suitable precursor for depositing Hf-relatedmaterials. Si(OnBu)4 was used as both Si and O

precursor [2]. Instead of using a separate oxygensource, silicon-alkoxide precursor was used in thiswork. Because the Si–O bonds are strong andshort, silicon alkoxides would be less oxidizingtoward silicon than water, hydrogen or ozone andthus can depress the formation of low-k interfaciallayers [2]. By using this new combination ofprecursors, we could deposit Hf-silicate films withimpurity concentrations less than 0.1 at% (belowdetection limits). Another advantage of this workis no requirement of additional oxygen precursorto grow silicate films.

2. Experiment

Hf-silicate films were deposited on 10Ocmp-type Si(1 0 0) (boron concentration 1015 cm�3)substrates at 400�C by MOCVD. Immediatelyprior to depositon, wafers were dipped in a diluteHF (HF:H2O=1:7) solution to remove any nativeoxide layers. During deposition, the pressure in thereactor was maintained at 0.1 Torr. The tempera-tures (vapor pressures) of Hf(NEt2)4 and Si(O

n-

Bu)4 bubblers were fixed at 70�C (0.7 Torr) and

95�C (2Torr), respectively. The transport lineswere kept at 100�C to avoid condensation ofprecursors. Argon (99.9995%) was used as abuffer flow to keep flow uniform and carrier gasto transport source vapors into the reactor. The Arflow rates of Hf(NEt2)4 and Si(O

nBu)4 were 3 and15 sccm, respectively. The buffer flow rate was15 sccm.Film thickness was measured by ellipsometry at

the wavelength of 370–1000 nm. Surface morphol-ogy was examined with scanning electron micro-scopy and atomic force microscopy (AFM). High-resolution transmission electron microscopy(HRTEM) was used to investigate the cross-sectional structure of the silicate/Si interface. Thechemical composition of the films was character-ized by Auger electron spectroscopy and X-rayphotoelectron spectroscopy (XPS) with Al Karadiation. Grazing incidence X-ray diffractometer(XRD) with Cu Ka radiation was employed toinvestigate the crystalline properties of the films.For electrical characterization of the silicate films,capacitance–voltage (C2V ) and current–voltage

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J. Kim, K. Yong / Journal of Crystal Growth 263 (2004) 442–446444

(I2V ) measurements were performed. The C2V

characteristics were analyzed at high frequency(1MHz) using HP 4275 multifrequency LCRmeter with a sweep voltage range of �8 to 8V.

Fig. 2. XPS depth profile of a 63 nm-thick hafnium silicate/Si

sample annealed at 600�C.

3. Results and discussion

Fig. 1 shows an HRTEM image of the Hf-silicate film grown on Si substrate at 400�C. Theinterface of silicate/silicon was seen to be atom-ically sharp with no interfacial silicon oxide layer,and the silicate was completely amorphous. AFManalysis of the surface morphology revealed a rootmean square (rms) of 3.1 (A indicating a verysmooth film.Fig. 2 shows an XPS depth profile of a 63 nm

Hf-silicate film annealed at 600�C for 1 h in O2ambient. The Hf/(Hf+Si) ratio [X in (HfO2)X(SiO2)1�X] was average 0.38 for the bulk film,indicating Si-rich composition. However, at thesilicate/silicon interface, it shows a higher Hfconcentration than Si, implying Hf-rich composi-tion in a few (A interface layers. The chlorine andnitrogen contaminations were less than 0.1 at%(below detection limits). Our preliminary experi-

Fig. 1. High-resolution TEM image of an interface between

silicon substrate and hafnium silicate film grown by MOCVD

using Hf(NEt2)4 and Si(OnBu)4 precursors at 400

�C. Due to

edge effects, the substrate in interfacial edges looks bright.

ments showed that Hf(NEt2)4 easily decomposeson Si(1 0 0) even below 300�C but Si(OnBu)4 doesnot decompose at 400�C. However, Si(OnBu)4 wasreacted to grow silicate films in the presence ofsmall amounts of Hf(NEt2)4. From these results,we believe that Hf metal catalyzes the C–O bondscission of Si(OnBu)4, These reactivity differencesof precursors and catalytic effects might induceHf-rich phases in the initial growth steps.The crystallinity of as-deposited and annealed

Hf-silicate films was investigated using grazingincidence XRD (Fig. 3). The (HfO2)0.38(SiO2)0.62film was annealed at 600�C, 800�C, 900�C and1000�C for 30min in flowing oxygen of 200 sccm.The Hf-silicate films were amorphous up to 800�Cannealing. Amorphous dielectrics resisting recrys-tallization up to high temperature are desirable forgate dielectric applications because grain bound-aries in polycrystalline films enhance the diffusionof dopants from the electrode to the substrate andmay exhibit high leakage paths [7,14]. Phaseseparation of the films occurred into amorphousSi-rich silicate and crystalline HfO2 phases at900�C.Fig. 4 shows C2V results obtained from a

24 nm (HfO2)0.1(SiO2)0.9 film. The film was an-nealed at 600�C for 30min in oxygen ambient.

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Fig. 3. XRD patterns of hafnium silicate films before and after

thermal treatments at various temperatures: as-dep, 600�C,

800�C, 900�C and 1000�C.

Fig. 4. High-frequency (1MHz) C2V curve of thin hafnium

silicate film O2-annealed at 600�C for 30min with an Al top

electrode (area=5.11� 10�4 cm2). The inset shows the corre-sponding I2V curve.

J. Kim, K. Yong / Journal of Crystal Growth 263 (2004) 442–446 445

Capacitors were fabricated using ex situ thermalevaporation of aluminum (Al) for electricalcharacterization. Area of aluminum gate electro-des was 5.11� 10�4 cm2. The gate voltage wasswept from accumulation to inversion and viceversa. The maximum capacitance in accumulationwas about 0.3 mF/cm2, which corresponds to k of

B8 (teq of 11.7). The attained dielectric constant islower than previously reported values of Hf-silicates (13–20) [5]. We believe that the low k

value in this study is due to low concentration ofHf in grown silicate films compared to previousworks. Hysteresis was about 0.1V. This very lowvalue of hysteresis is due to low impurity anddefect contents in our silicate films [15]. Also, theslope of C2V curve near by center was very steep,indicating a low density of interface states. TheI2V characteristics for the same Hf-silicate areshown in the inset of Fig. 4. The leakage currentdensity was 2.2� 10�8A/cm2 at a bias of 1.0V.

4. Conclusions

Si-rich Hf-silicate films have been deposited byMOCVD using a new combination of precursors:metal complex of amido and alkoxide groups. Wecould dramatically reduce chlorine and carboncontaminations, which are common impurities inCVD or ALCVD when using metalorganic pre-cursors. An atomically flat interface with no SiO2interface layers was observed. The grown Hf-silicate films were amorphous up to 800�Cannealing with no crystallization and phaseseparation. The grown Hf-silicate films have adielectric constant ofB8 with a very low hysteresisof 0.1V in C2V curve. The leakage currentdensity was 2.2� 10�8A/cm2 at a bias of 1.0V.

Acknowledgements

This work was supported by grant No. R01-2002-000-00279-0(2002) from Korea Science &Engineering Foundation and Postech ResearchFund.

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