Carboxylic Acids as Oxygen Sources for the Atomic Layer Deposition of High-K MetalOxides

Erwan Rauwel,† Marc-Georg Willinger,† Frederique Ducroquet,‡ Protima Rauwel,§Igor Matko,| Dmitry Kiselev,§ and Nicola Pinna*,†

Departments of Chemistry and Ceramics, CICECO, UniVersity of AVeiro, 3810-193 AVeiro,Portugal and IMEP, UMR CNRS 5130 and LMGP, UMR CNRS 5628, INPG-Minatec, BP 257,38016 Grenoble cedex 1, France

ReceiVed: April 29, 2008; ReVised Manuscript ReceiVed: June 3, 2008

A nonaqueous approach inspired from sol-gel chemistry and adapted to the deposition of metal oxide thinfilms by atomic layer deposition (ALD) is presented. The process is based on the reaction of a carboxylicacid with a metal alkoxide. Contrary to classical approaches, no oxygen source that could lead to the oxidationof the silicon substrate is required. Instead, a surface esterification reaction is responsible for the film formation.The growth of metal oxides is achieved at temperatures as low as 50 °C on various supports including carbonnanotubes, organic fibers, and silicon wafers. The as-grown films show an excellent conformality and possessgood dielectric properties due to their high purity. Inherent to the chemical approach is the possibility togrow oxides on silicon while minimizing the formation of a low-κ interfacial layer.

Introduction

Synthesis of nanometric thin films with desired compositionsand uniform coverage holds a central position in nanotechnologyresearch. As the deposition of oxide thin films on many differentsupports appears to be a real challenge, new deposition proces-ses are constantly investigated. Among all of the depositionprocesses, atomic layer deposition (ALD)1–3 appears to be oneof the most promising techniques due to its simplicity, repro-ducibility, and the high conformality of the obtained films. ALDis based on a reaction between precursor materials that areseparated into successive surface reactions. In this manner, thereactants are kept separated until the adsorbed species react atthe surface in a self-limiting process,1,4,5 that is, without thepresence of a gas phase reaction.

ALD is already being used in the fabrication of microelec-tronics devices on a large scale. The complementary metal oxidesemiconductor field effect transistor (CMOSFET) based onsilicon technology is presently the most important device inmicroelectronics for its performance and low power consump-tion. However, due to downscaling of metal oxide semiconduc-tor (MOS), the International Technology Roadmap for Semi-conductors (ITRS) projects that CMOS technology will nowrequire gate dielectric layers with higher dielectric constantsthan SiO2 or SiON6 in order to reduce the tunneling leakagecurrent.7,8 Many high-κ materials have been investigated for thereplacement of SiO2 in high performance CMOS.9,10 In thisrespect, considerable attention has been given to HfO2 due toits relatively high dielectric constant,11 wide band gap,12 and agood thermodynamic stability with silicon.13 Titanium dioxidealso has very attractive dielectric properties despite its lowerband gap.10 However, it is more known for its photocatalyticproperties and its high refractive index, making it suitable for

optical coatings and as white pigment. Alternatively to Si-basedtechnology, it was recently demonstrated that the deposition ofhigh-κ metal oxides on carbon nanotubes (CNTs) can be usedto fabricate new metal oxide field effect transistors.14–16

Usually, the deposition of metal oxides by ALD is achievedby using an oxygen source (e.g., water, oxygen, ozone) with ametal precursor (e.g., halide, alkoxide, amide).17 Generallimitations of this traditional method mainly reside in thepresence of undesired impurities, ranging from unreacted carbonspecies to halides, and in the relatively high growth temperatures(200-500 °C).18,19 Depending on the targeted application, atailored postsynthetic treatment is generally required in orderto improve the quality of the films. Even though Chabal et al.have shown that H-terminated Si(001) is stable in the presenceof water for temperatures up to 300 °C,20–22 metal oxide filmsgrown on silicon by ALD processes employing “traditional”oxygen sources systematically present an oxidized interfaciallayer (silica or silicates).

In the search of alternative approaches making use of less-oxidizing conditions, nonaqueous sol-gel routes seem to beappropriate because the metal-oxygen-metal (M-O-M)bonds can in certain cases be formed without a hydrolysis step.Instead, the oxygen is provided by other molecular species suchas alcohols, ethers, or metal alkoxides.23,24 These routes wereextensively applied to the synthesis of nanoparticles becausethe as-synthesized oxides display high crystallinity and purity.25–27

They were also applied to ALD, but up to now they did notprove to bring significant advantages compared to approachesmaking use of water or other oxidizing agents.28,29 Recently,we showed that the ALD of metal oxides from carboxylic acidsand metal alkoxides takes place via an ester eliminationcondensation step.30 The reaction mechanism taking place duringthe deposition was deeply studied and discussed in the samearticle. In this article a detailed study of titania and hafnia thinfilms deposited on various supports according to this approachwill be presented.

* To whom correspondence should be addressed. E-mail: pinna@ua.pt; fax: +351 234370004.

† Department of Chemistry, CICECO, University of Aveiro.‡ IMEP, UMR CNRS 5130, INPG-Minatec.§ Department of Ceramics, CICECO, University of Aveiro.| LMGP, UMR CNRS 5628, INPG-Minatec.

J. Phys. Chem. C 2008, 112, 12754–1275912754

10.1021/jp8037363 CCC: $40.75 2008 American Chemical SocietyPublished on Web 07/18/2008

Experimental Methods

Thin films were grown by atomic layer deposition in an ALDdeposition system working in exposure mode. They weredeposited onto p-type Si(001), p-type Si(111), carbon nanotubes,wool fibers, cellulose fibers, and latex. Some of the siliconsubstrates were etched in hydrofluoric acid (HF 3%) for 2 minprior to the deposition (labeled “HF-last”) in order to removethe native silicon dioxide.

Films were deposited using the following metal alkoxides:hafnium tert-butoxide (STREM, 99.9%) and titanium isopro-poxide (Aldrich, 99.99%). Acetic acid (Fluka, > 99.8%) wasused as oxygen source. Pure nitrogen was used as carrier andpurging gas. The substrate temperature was varied from 50 to350 °C. Metal precursors and acetic acid vapors were generatedin external reservoirs preheated at 80 and 40 °C, respectively.They were introduced into the reactor through an ALD valve.All tubes of the circuit were maintained at 100 °C during thedeposition process. Depending on the precursors used, thenitrogen gas flow was varied from 5 to 50 sccm and wasmaintained throughout the deposition process. In a typicalexperiment, the valves were opened for 0.02 s for carboxylicacid and 1 s for the metal alkoxides. The residence and purgingperiods were 20 and 15 s, respectively. CNTs from AppliedScience were treated with concentrated HNO3 at 100 °C for2 h before deposition.

The thickness of the films was determined by X-ray reflec-tometry (XRR) measurements using a Philips X’Pert MPD X-rayDiffractometer with a copper radiation (λKR ) 1.54056 Å) anda graphite monochromator for the selection of pure KR radiation.The X-ray tube was operated at 40 kV and 50 mA. A 1 mm slitwas used to reduce the scattered X-ray intensity. Reflectiongeometry was used in the measurement. Instrumental config-uration: divergence slit at the incident beam, equal 1/8 in.; stepwidth, 0.01°; acquisition time, 0.6 s.

High resolution transmission electron microscopy (HRTEM)on thin films was carried out on a JEOL 2010 transmissionelectron microscope operating at 200 kV and equipped withLaB6 filament with a point-to-point resolution of 1.7 nm;measurements on nanotubes were performed using a CM200FEG(Philips) microscope operating at 200 kV and equipped with afield emission gun and post column electron energy lossspectrometer (GATAN Tridiem). Scanning electron microscopy(SEM) images were recorded using a FEG-SEM Hitachi SU-

70 microscope operating at 4 kV with a working distance of2-3 mm. For SEM, samples were prepared without any carboncoating simply by depositing some powder onto a double gluingtape.

The electrical properties of the dielectric thin films werestudied in metal oxide semiconductor (MOS) structures byevaporation of gold electrodes through a hard mask. C-V curveswere recorded with a HP4284A LCR meter using a 40 mVsignal at a frequency ranging from 0.1 kHz to 1 MHz. The stair-sweep voltage step was 20 mV with a dwell time of 2 to 4 sbetween the steps. All measurements were performed at roomtemperature, on as-deposited thin films without any subsequentthermal or chemical treatment.

Atomic force microscopy (AFM) measurements were per-formed at room temperature with a commercial scanning forcemicroscope (PicoPlus, Agilent Technologies). A commercialcantilever (PPP-NCHR Nanosensors) with spring constant of k) 42 N/m and typical tip radius less 10 nm of the tip was used.Imaging visualization was performed in air with noncontactmode (tapping mode with resonance frequency at 275 kHz) atthe scanning speed 1 µm/s.

Results and Discussion

The deposition of titania and hafnia according to the presentapproach shows a growth per cycle (GPC) that increase withtemperature (Figure 1a and 2a). For titania, a typical ALDwindow is observed between 150-200 °C (Figure 1a). In thattemperature range, the self-limiting character of the process isevidenced by the saturation of the GPC (0.06 nm) independentof the quantity of metal precursor introduced into the reactor(Figure 1b). Moreover, the observed GPCs are in agreementwith those generally observed.31,32

In the case of hafnia, the GPC is larger, although no clearplateau (e.g., ALD window) can be identified. Between 200 and300 °C the GPCs are in the same range as those obtained in thecase of hafnium alkylamides (0.09 nm)33 but are higher thanthose observed when hafnium halides and water are used.34 TheGPC as a function of the valve opening time is plotted for areaction taking place at 200 °C (Figure 2b). It was found thatthe GPC saturates and does not further increase for openingtimes exceeding 0.5 s. This proves the self-limiting characterof the process at this temperature.35

Figure 1. (a) GPC of titania thin films deposited on silicon substrate plotted as a function of temperature. At 300 °C the GPC is dependent on theopening time of the alkoxide valve (square 1 s, star 1.5 s, triangle 2 s). (b) GPC as a function of the opening time of the alkoxide valve at 200 °C.

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Depositions taking place at 300 °C (i.e., above the region ofself-limiting growth) correspond to a region of diffusion-controlled growth. It results from the self-decomposition of themetal precursors. In that regime, the GPC can be modulated bythe amount of metal alkoxide introduced (Figure 1a and 2a,experimental points at 300 °C)

Atomic force microscope (AFM) studies indicate that thefilms exhibit a smooth surface with a low root-mean-square

(rms) roughness of around 0.2-0.3 nm. An 11 nm thick HfO2

film deposited on silicon, for example, shows a rms roughnessof only 0.27 nm, corresponding to a relative thickness of 2.70%(Figure S1, Supporting Information).

The high resolution transmission electron microscopy (HR-TEM) images recorded from cross section preparations of HF-last Si(001) and Si(111) wafers coated with titania or hafniaare shown in Figure 3. In all the cases a high conformal and

Figure 2. (a) GPC of hafnia thin films deposited on silicon substrate plotted as a function of temperature. At 300 °C the GPC is modulated by theopening time of the alkoxide valve (square 1 s, triangle 1.8 s). (b) GPC as a function of the opening time of the alkoxide valve at 200 °C.

Figure 3. HRTEM images of (a) TiO2 grown at 200 °C on Si(001) substrate with an interfacial layer of ∼0.4 nm, (b) TiO2 grown at 200 °C onSi (111) substrate with no evidence of an interfacial layer, (c) HfO2 grown at 200 °C on Si(001) substrate presenting an interfacial layer thicknessof ∼0.7 nm, and (d) HfO2 grown at 80 °C on Si(001) substrate presenting an interfacial thickness of ∼1.15 nm. In the panel (a) the circled numbersdenote the silicon wafer (1), the interfacial layer (2), and the metal oxide film deposited (3), respectively.

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generally homogeneous metal oxide film was detected. In theTEM, the high-κ films appear amorphous, as a matter of fact,they did not show lattice planes that could denote the presenceof randomly oriented nanocrystallites or epitaxial layers. Themost remarkable findings concern the sharp and thin interfacebetween the silicon substrate and the metal oxide thin films.Especially in the case of TiO2 deposited on Si(111) at 200 °C,no oxide interlayer was detected (Figure 3b). For TiO2 thin filmsgrown on Si(001) under the same conditions, only a very thinlow-κ interfacial layer (<4 Å) was observed (Figure 3a).Concerning the hafnia, an oxide interlayer always remained forfilms deposited on Si(001) (Figure 3c). The layer thicknessmeasures around 0.7-1.2 nm for a growth temperature of 200°C and roughly corresponds to the native surface oxide ofunprocessed Si wafers. At lower growth temperatures (80 °C),the interfacial oxide layer still remained relatively thin (1.15nm), even for a 33 nm thick HfO2 film (Figure 3d). TEM resultsreveal that the process is capable of forming high conformalfilms even at low growth temperatures, although the homogene-ity of the obtained film is slightly lower in that case (Figure3d). On the basis of the available data it is difficult to explainwhy titania can be directly deposited on silicon whereas a thininterfacial low dielectric layer is always observed in the caseof hafnia. A closer look at the chemical processes taking place

during the deposition demonstrates that similar chemical reac-tions are responsible for the metal oxide formation in bothcases.30 Hence, they are not accountable for this behavior.However, it is evident that even small differences in the chemicalproperties of the metal precursors can give rise to a dissimilargrowth of the interfacial layer. Detailed studies on the effect ofmetal alkoxides presenting different alkyl groups or the use ofin situ techniques such-as infrared spectroscopy36 could helpto understand the formation of the interfacial layer.

To the best of our knowledge, ALD proved to achieve metaloxide growth on silicon without an interfacial layer only in thecase of alumina, which is deposited under nonaqueous sol-gelconditions.28,37 The group of Ritala et al.28 reacted AlCl3 withAl(OiPr)3 as oxygen source at 300 °C and Jeon et al.37 achievedit with Al(OiPr)3 and isopropyl alcohol at 250 °C.

Certainly, one of the novelties of the current approach is thatit enables the production of thin films that show remarkableelectric properties, even for depositions at low temperature andwithout any postdeposition treatments. The capacitance-voltage(C-V) measurement of as-deposited 10 nm thick HfO2 filmgrown at 200 °C on Si(100) shows a well-shaped C-V curve(Figure 4). After subtraction of the interfacial layer contribution(estimated from the TEM cross sections) from the overallcapacitance, the hafnia film exhibits a relative permittivity of23, which is in good agreement with those commonly foundfor pure amorphous HfO2. The high quality of the HfO2 film isalso supported by the presence of only a weak hysteresis andby a flat band voltage close to the ideal value (the ideal value

Figure 4. Capacitance-voltage curve of a 10 nm thick HfO2 filmdeposited at 200 °C on (100) Si substrate. The arrows indicate thevoltage sweep direction: full symbols are from depletion to accumula-tion and empty symbols are from accumulation to depletion. In theinset, current-voltage characteristics of MOS structures with a 10 nmthick HfO2 film deposited at 200 °C on (100) Si substrate and (a) 0.7nm interfacial layer, (b) 1.2 nm interfacial layer, (c) 33 nm thick HfO2

film deposited at 80 °C.

Figure 5. TEM images of parts of carbon nanotubes coated with (a) HfO2 and (b) TiO2.

Figure 6. EELS spectra of titania-coated CNTs acquired in the center(solid line) or at the boarder (dotted line) of the tube in Figure 5b.

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for HfO2 is 0.22 V under the consideration that the gold workfunction equals 5.1 V). These results indicate a very low levelof trapped and fixed charges in this film. A small contributiondue to interface states is indicated by the capacitive shoulderpresent in the depletion region. The accumulation current versuscharacteristic voltage also exhibits a low leakage current density(2.5 × 10-6 A/cm2 @ -1 V) (inset of Figure 4). A slightincrease of this current is observed for HfO2 films with a thinnerinterfacial layer (0.7 nm). It should be noticed that even for the33 nm thick HfO2 film deposited at 80 °C, the accumulationcurrent remains low (6 × 10-7 A/cm2).

To confirm the versatility of the process, CNTs were alsocoated with hafnia and titania. TEM images of the tubes areshown in Figure 5. The darker regions on the outer and innerwalls of the CNTs correspond to the metal oxide layersdeposited by the process. The coating is uniform along the wholesurface of the nanotubes and presents approximately the samethickness in the inner and outer surface. The fishbone-like gra-phitized walls are well-resolved in the high resolution imagerecorded from a tube coated with a thin layer (∼ 3 nm) of HfO2.HRTEM and selected area electron diffraction measurementsprove that for both materials the coating layer is amorphous(see also Figures S2 and S3 in the Supporting Information). Toprove the chemical composition and purity of the depositedmetal oxides, electron energy loss spectrometry (EELS) wasused. Spectra recorded from the sole film on the outer surfaceand another one recorded from film and tube in the central regionof the tube are shown in Figure 6. The carbon signal observedin the spectrum recorded from the surface region of the tube isweak. Because of the high ionization cross section for the carbon1s electrons, EELS is very sensitive to carbon. This is evidentfrom the spectrum recorded from the central region of a tube,where the integrated area of the carbon signal dominates thespectrum. On the surface region of the coated tube, the spectrumshows a small carbon signal. Elemental quantification abstractedfrom that spectrum reveals a carbon content of less than 5%.This is a rough estimation of the upper limit of carbon in thefilm, and one has to take into account the close proximity ofthe tube walls and an eventual collection of some signal fromthe tubes walls (i.e., through a slight drift of the sample dur-ing the spectrum acquisition). These results corroborate previousX-ray photoelectron spectroscopy (XPS) studies performed onthin films deposited on silicon, where a very low amount ofcarbon contamination was found.30

A wide range of different support materials such as wool,cellulose fibers, and natural latex have been coated with theaim of studying the adaptability of this process to other fields.Figure 7a shows wool fibers coated with titania, proving that

the process does not modify or damage the surface structure ofthe fibers and therefore allows a replication of the fineststructural details of the material (cf. Figure S4, SupportingInformation). Titanium and oxygen energy-dispersive X-rayspectroscopy (EDX) maps superimposed to the image in Figure7a shows that the fibers are homogeneously coated with titania(Figure 7b).

Conclusions

In summary, the benefits and potential applications of arecently introduced nonaqueous sol-gel route30 for the atomiclayer deposition of high conformal thin films was demonstrated.The main benefits of employing carboxylic acid as oxygensource are their less oxidative nature as compared to traditionaloxygen sources and the possibility to grow films at relativelylow temperature on various substrates. These are essentialprerequisites for the growth of films on substrates that aresensitive to strongly oxidizing conditions or/and to hightemperatures. This was demonstrated by growing titania filmswithout the formation of a silica interface and by coating organicfibers. Moreover, the quality of the films is high, as evidencedby the dielectric properties and the low carbon content. Finally,it was shown that carbon nanotubes can be coated even on theinside, proving the high conformality of the obtained films andthe versatility of the process. This opens the door for applicationsin various fields such as microelectronics or (photo)catalysisand the fabrication of advanced multifunctional nanomaterials.

Acknowledgment. The authors would like to thank ProfessorDr. Wolf Widdra and Ralf Kulla from the Institute of Physicsof the Martin Luther University, Halle, Germany, for supporton the conception and realization of the ALD tool; Dr. BernardChenevier LMP-Minatec, Grenoble, France for the use of theJEOL 2010 TEM; Dr. Dangsheng Su and Rosa Arrigo fromthe Fritz Haber Institute of the Max Planck Society, Berlin,Germany, for supplying carbon nanotubes; and Dr. Igor Bdikinfrom the department of Ceramics, CICECO for the AFM study.Financial support from FCT project No. PTDC/CTM/65667/2006 and grant No. SFRH/BPD/36941/2007, FAME networkof excellence, and Marie Curie (MEIF-CT2006-041632) isacknowledged.

Supporting Information Available: Additional materialscited within the text are provided. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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Figure 7. (a) SEM and (b) titanium and oxygen SEM EDX mapping of wool fibers coated with titania.

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